In a thermal printer, a cyan, magenta or yellow dye-donor element is placed face-to-face with a dye-receiving element. The two are then inserted between a thermal printing head and a platen roller. A linear array thermal printing head is used to apply heat from the back of the dye-donor element. The thermal printing head has many heating elements and is heated up sequentially in response to appropriate cyan, magenta or yellow electrical signals. The process is then repeated for the other two colors. Further details of this process and an apparatus for carrying it out are contained in U.S. Pat. No. 4,621,271.
Thermal prints are susceptible to retransfer of dyes to adjacent surfaces and to discoloration by fingerprints because dye is present at the dye-receiving surface of the print. Application of a protection overcoat material will practically eliminate these problems.
In a thermal dye transfer printing process, it is desirable for the finished prints to compare favorably with traditional silver-halide color photographic prints in terms of image quality. The look of the final print is very dependent on the surface texture and gloss. Typically, color photographic prints are available in surface finishes ranging from very smooth, high gloss to rough, low gloss matte. However, applying a thermal image to a rough surface would result in uniformity problems and dropouts. If a matte finish is desired on a thermal print, it has been previously been accomplished by using matte sprays or by matte surface applications through post printing processors. However, both of these solutions are costly and add a degree of complexity to the process.
Commonly assigned U.S. Pat. No. 6,184,181 discloses a process for controlling the gloss of a thermal dye transfer image by applying a protection overcoat layer on top of the transferred dye image, the protection layer being applied from an element that contains unexpanded synthetic thermoplastic polymeric microspheres. The protection layer is transferred using a predetermined energy level in order to expand the microspheres until a desired gloss level is obtained.
U.S. Pat. No. 6,092,942 relates to the formation of a silk texture shaped concave-convex pattern on a transparent cover sheet as it is transferred to the printing medium. U.S. Pat. No. 6,382,852 discloses controlling a thermal head so that a specific amount of thermal energy is applied to a recording surface. The thermal energy effects variations of glossiness corresponding to additional information to record. That is, through the application of the thermal energy, glossiness of the medium varies and the additional information is thereby recorded on the medium, and textual information such as a date and time may be recorded as additional information through the use of glossiness.
The printing of high quality black and white and multiple color images on relatively glossy print media together with identification indicia, such as alphanumeric characters or spacing marks to identify the printed image, its date of printing and other information is well known in the prior art. For example, in photography, it is known to image latent frame numbers and marks on unexposed film, as taught, for example, in U.S. Pat. No. 4,553,833. The latent image becomes visible after processing. In photographic printing, it is known to expose spacing marks along the sides of images printed on continuous rolls of photographic print media to mark the reverse surfaces of photographic prints with the date of printing. It is also known to expose areas of X-ray film with latent-alphanumeric identification characters and other information applied by thermal elements energized by microcomputer-stored image data as taught, for example, in U.S. Pat. No. 4,983,991, where the latent image characters become visible after processing the film. In thermal printing systems which effect a printing operation through heat transfer of dyes to a printing medium, it is known to print images or indicia and markings by dye transfer to the printing medium as taught, for example, by U.S. Pat. No. 4,710,781, U.S. Pat. No. 4,995,741, and U.S. Pat. No. 4,516,137. It is also known from the '137 patent to employ heat sensitive media that changes color on application of heat and pressure from the thermal elements of a printhead.
In many cases, a printer or print engine is connected to a variety of image generating sources networked together so as to allow for transmission of images from the sources to the printer or print engine. These images are received by the print engine and printed in accordance with a prioritizing scheme on an elongated element of thermally activated dye transfer printing medium. As a result of the prioritizing scheme, sets of interrelated images may be distributed out of sequence on the printed web media over a period of time. Alternatively, if the printing medium is in sheet form, the individual printed sheets may accumulate out of order in one or more bin. In either case, the operator may be required to collate the related images and distribute them to various areas or customers. It is thus desirable that the printed images be individually identifiable for such distribution. It is advantageous to the operator to have the prints labeled with indicia identifying the source, content and/or distribution, as well as the date of printing. It is also advantageous for this information to be visually apparent to the trained operator but unobtrusive in other respects.
In respect to the above-mentioned thermal printing systems, dye transfer is effected from a dye-donor element interposed between the thermal printhead elements and the printing medium in response to the applied thermal energy. Alternatively, thermally responsive print media is designed to change color upon application of minute amounts of thermal energy. In the former case, the separate dye transfer web suffers the drawbacks of the impact printer web, and in the latter case, the thermally responsive print media may be incompatible with the image bearing media.
It was therefore desirable to provide a printing system for printing unobtrusive indicia on preexisting media that is simple and relatively failsafe. Commonly assigned U.S. Pat. No. 5,369,419 discusses how such marking may be accomplished by selectively altering the given specularity so as to create alphanumeric characters or other markings that are observable by the contrast between the altered specularity and the given specularity by viewing the same with light reflected from opaque media or transmitted through transparent or semi-transparent media. Heat patterns are applied to a marking area of given specularity to effect a change in that specularity, typically from dull to highly glossy, visible to the viewer. Thus, a sheet having a low gloss, low specularity, matte surface is provided with high gloss characters. Similarly, U.S. Pat. No. 7,126,618, describes printing apparatus forms image information and information associated therewith on a print medium. The printing apparatus receives image information from an image file stored in a recording medium, etc., and obtains information associated with the image information. The associated information includes attached information that is read out from the image file based on a file format using tags via an interface unit or added information that is input or selected in accordance with the image information. The image information is printed on the print medium, and the associated information is formed on a film sheet (laminate film), which is disposed over the image-printed surface, as a watermark. The associated information is not visible when the print medium is seen from the front, and can be seen when it is seen at an angle due to the difference in surface glossiness.
Although the method and apparatus disclosed in above-described U.S. Pat. No. 5,369,419 works well for its intended purpose, it has been discovered that marking some textured surfaces with a high gloss pattern produces characters that are rather difficult to read under many lighting situations. Accordingly, it is an object of the present invention to provide a matte surface with more discernable markings.